Generally, hot rolled section steels are bent to such a degree that they can not be used directly because of thermal shrinkage and the like due to nonuniformity of cooling temperature across the section of said section steel, for instance, during rolling or in the cooling process after the finish of forming, so that it is common practice to straighten said bent steel after cooling thereof by means of a straightening mill.
And, as is generally known, in the straightening mill the bent steel is passed between upper and lower straightening rollers disposed to define a pass line therebetween. By passage of the steel there-through the bent steel may be satisfactorily straightened except for the front portion and the tail portion.
But, in the vicinity of front and tail ends of the steel material to be straightened, there occurs deformation of the steel material which is different from the deformation of the intermediate portion between the ends and this is regarded as the reason why the bending which is called "end crook" remains at the front portion and the tail portion.
This "end crook" creates a difficult problem when it is desired to join together the same kinds of sections or different sections, for instance, as in the case of rails for railway trucks which are used in series. Thus, it is considered as a serious defect in the quality of the section steel, in which straightness of the top surface is considered an important quality. However, with conventional mills, repeated passes are necessary in order to deal with end crook. However, the productivity is considerably hindered, not only because of low working operation of a press, but also because secondary working is required.